Satellite radio operators are presently providing digital radio broadcast services covering the entire continental United States and other parts of North America. These services offer approximately 170 channels, of which nearly 75 channels in a typical configuration provide music and the remaining channels offer news, sports, talk and data services. A block diagram of an illustrative satellite digital audio radio service (SDARS) system 10 is depicted in FIG. 1. The illustrative SDARS system is a diversity system in which time, spatial and or code diversity is employed to overcome signal losses. For example, SDARS receivers demodulate and decode broadcast streams from multiple transmission sources such as first and second satellite streams broadcast from first and second satellites for time and spatial diversity purposes and/or terrestrial broadcast streams (e.g., from such terrestrial transmission sources as terrestrial repeaters, paging systems and/or cellular systems) employed to overcome LOS issues and other signal loss issues described below. For example, an SDARS system operated by Sirius XM Radio Inc. includes satellite uplink stations 2a, 2b for transmitting X-band uplinks to two satellites 4, 6 which provide frequency translation to the S-band for retransmission to radio receivers 3 within a coverage area. Radio frequency carriers from one of the satellites 4, 6 are also received by terrestrial repeaters 5. The content received at the terrestrial repeaters 5 is retransmitted at a different S-band carrier to the same receivers 3 that are within their respective coverage areas. These terrestrial repeaters 5 facilitate reliable reception in geographic areas where line of sight (LOS) reception from the satellites 4, 6 is obscured by tall buildings, hills, tunnels and other obstructions. The signals transmitted by the satellites 4, 6 and the repeaters 5 are received by SDARS receivers 3 which can be located in automobiles, in a handheld unit or in stationary units for home or office use. The SDARS receivers 3 are designed to receive one or both of the satellite signals and the signals from the terrestrial repeaters, and combine selected signals or select one of the signals as the receiver output. Thus, the receivers 3 can demodulate, decode and output a selected channel from the received signals even when, for example, a signal dropout has occurred in one of the transmission channels.
In a legacy SDARS system implemented by Sirius XM Radio Inc. described above, the plurality of base layer services can be modulated using a Quadrature Phase Shift Key (QPSK) modulation technique. As shown in FIG. 2, four constellation points are possible based on the combinations of the two symbols to be transmitted. These points are located at 45°, 135°, 225° and 315° as shown as (BL,X), where BL indicates the base layer symbol in the transmitted base layer QPSK constellation using 2 input BL bits.
The above base layer modulation technique can be enhanced to carry additional information by implementing a technique called hierarchical modulation. Hierarchical modulation can be applied to the QPSK transmission to modulate an additional symbol or symbols to the transmission by further modifying the base layer QPSK transmission. Hierarchical modulation enables the multiplexing and modulating of a plurality of data streams into a single data stream by overlaying the additional information onto a base layer. A need exists for realizing additional data capacity by improving or enhancing hierarchical modulation techniques to provide unique opportunities to enhance legacy SDARS services or other legacy broadcast services of systems that transmit data using diversity streams. A need also exists for using improved (next generation) receiver designs such that additional services can be provided to users while existing legacy receivers can continue to receive the services broadcast on the base layer modulated data stream in the legacy system.
Some examples of hierarchical modulation schemes on a QPSK waveform, which can be employed in an illustrative embodiment of the present invention described below, are shown in FIGS. 2a and 2b. Constellation (a) illustrates a phase shift keying (PSK) modulation technique to overlay the additional information onto a base layer. In this technique, the received vector is mapped as (BL, OL). The OL bit indicates an overlay symbol when the base layer modulation vector is rotated or phase offset by a predetermined angle +/−dφ toward either the Q-axis or the I-axis. As shown, for example, if the base layer modulation vector is rotated toward the Q-axis, the OL bit is represented by a 1, and if the base layer modulation vector is rotated toward the I-axis, the OL bit is represented by a 0. Where the OL bit is designated with an ‘x’, there is no rotation performed and therefore there is no overlay modulation. As shown in this example, for every two base layer bits transmitted, an additional bit can be overlaid onto the base layer.
Legacy receivers in the SDARS system, however, expect to receive a QPSK signal, thus the hierarchically modulated phase offset appears as an unnatural noise enhancement. In other words, the addition of a phase offset in the QPSK modulation appears as a phase error with respect to the base layer QPSK modulation. Transmission of the hierarchically modulated data on a noisy channel with a low signal-to-noise ratio may prohibit the detection of the base layer QPSK signal, much less the desired phase offset of the overlay layer modulation. Under certain conditions, the phase offset error can inhibit performance and synchronization of the received signal by impairing carrier recovery and tracking loops used to acquire and maintain receiver synchronization with the broadcast signal.
The terrestrial signals in an SDARS system are typically modulated utilizing a multi-carrier modulation technique such as Coded Orthogonal Frequency Division Multiplexing (COFDM). In legacy systems, this consists of multiple carriers, each modulated utilizing QPSK modulation. In accordance with an illustrative embodiment of the present invention described below, the hierarchical modulation of the overlay stream can be applied to the legacy signal via an APSK technique as shown in constellation (b), where the hierarchical modulation is added to the base layer modulation via amplitude modulation. In this method, the overlay bit is determined at a receiver by comparing the amplitude of the received vector with a reference amplitude. As shown, if the transmitted vector is produced with reduced amplitude scaling, the OL bit is designated as a 1, and if the transmitted vector is produced with increased amplitude scaling, the OL bit is designated as a 0. If there is no determined amplitude scaling with respect to the reference amplitude, there is no additional information overlaid onto the base layer.
Due to channel imperfections in a mobile environment, the amplitude of the signal of the receiver is affected by multi-path effects that result in amplitude variations, which differ across the COFDM sub-carriers and also differ in time as the receiver moves through the environment. As a result of the multi-path effects, the ability to discern amplitude modulation is greatly reduced and therefore it is very difficult to estimate the channel conditions and synchronize the received signals.
Accordingly, there is also a need for a method of improving the synchronization of a received hierarchically modulated signal, especially when legacy decoder synchronization is relied on for the retrieval of the overlay data as well; otherwise, the performance of both the legacy services, and the additional services achieved via hierarchical modulation, suffers.